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The structure and unsteadiness characteristics of a shock train in a Mach 2.0 ducted flow are studied. The shock train is generated and stabilized by a back-valve that imposes a desired back pressure. Despite nominally constant boundary conditions (i.e., inflow and outflow), the shock train fluctuates about its time-average position. High-speed schlieren imaging is used to quantify the amount of unsteadiness. The results show that the shock fluctuation speed and the magnitude of shock displacement are independent of back pressure. However, the angles of the leading shock lambda foot and the length of the leading shock Mach stem change as back pressure is increased indicating the shock structure transitions from oblique to normal as the shock train moves upstream. The structural changes of the shock train are reflected in time-averaged pressure profiles. As the shock train moves upstream the length of the shock train decreases and the pressure rise across the shock train increases. Finally, stereo particle image velocimetry is used to study the structure of the leading shock in a normal shock train (i.e., when the back pressure is high and pushes the shocks upstream). The velocity fields show a large separation region on the side-wall of the duct and a degree of axisymmetry that indicates a nominally conical shape. The overall structure of the leading shock is modeled as two truncated cones with the small ends coinciding to form the Mach stem.

Unsteadiness characteristics and three-dimensional leading shock structure of a Mach 2.0 shock train